I FUCKING DID IT !
It was so simple ! And I knew it was simple yet I was too stupid to find out the solution (and probably lacking some sleep) but I FINALLY DID IT !!!!

I wanted to bring back the much better looking End Portal effect that got replaced by the ugly screen coordinates one we have today though Mojang didn't make it easy for me
There was no vertex normals for the end portal so I had to recreate them in the shader
BTW it's all Vanilla Shaders, no mods